Processing corn grain to yield component parts of the grain as animal feed, human food and feedstocks for industrial processes such as fermentation to make ethanol may broadly be divided into two categories—wet milling and dry milling.
In a classical wet milling operation the corn grain is “steeped” (typically for 22-50 hours at about 50° C.) in an aqueous solution often including small amounts of a mild sulfur acid compound such as sulfur dioxide, hydrogen sulfide, sulfuric acid or calcium sulfate which loosens the pericarp (bran) tissue from the interior endosperm and more importantly, from the germ tissue. The water usage in a corn steeping operation is high, on the order of 5-9 gallons per bushel. The endosperm and germ tissue are separated from each other and from the loosened pericarp by filtration and differential floating in an aqueous solution involving additional inputs of water. Corn oil is extracted from the separated germ using a hydrophobic organic extractant such as hexane, leaving behind a protein enriched de-oiled germ cake useful as animal feed. Through a multi-step process involving further uses of input water, protein is separated from the separated pericarp and endosperm fractions forming a protein enriched product denoted “corn gluten”, which in various degrees of purity may be called corn gluten feed, or corn gluten meal. The residual pericarp tissue denoted “corn fiber” is typically used solely for animal feed purposes, while the highly purified starch granules of various dimensions from the endosperm may be used for a variety of purposes, including for example to make fermentation products, which are made by liquification and saccharification of the starch to form dextrose to feed the fermentation organism to make the fermentation product—for example, ethanol. The starch granules may also be separated into various sizes and or ground into human food products such as corn grits and corn starch. The final corn fiber, corn gluten, and corn gluten feed products are sold in the market for additives to animal feed.
In a conventional “dry grind” process less water is used and the equipment costs for a plant are less. Typically the corn is briefly “tempered” with a small amount of water without a sulfur compound to bring the grain to a moisture content of 14-20% to loosen the pericarp. The tempered grain is then subject to grinding to form coarse corn grain flour with the endosperm and germ exposed. Starch in the exposed endosperm tissue is saccharified and liquefied to form dextrose. The entire cracked and liquefied mixture including the pericarp and germ tissue is used for a fermentation process to yield, for example, ethanol. Ethanol is separated from the fermentation broth by distillation and the residual germ, pericarp, and undigested starch components of the grain, along with the yeast biomass made during the fermentation process are dried to form a product called “distillers dried grains” (DDGs) which are useful as an animal feed ingredient.
There are various modifications of the dry grind process denoted “modified dry grind corn” processing that may include additional or intervening steps such as aspiration, sieving, floatation and/or filtration to at least partially separate at least one of the pericarp and germ fractions from the endosperm fraction prior to saccharification for fermentation. These partially separated fractions are less pure than those obtained from a wet grind operation, but may be used for products similar to those obtained from wet grind process. For example corn oil may be obtained from the partially separated germ and the extracted germ cake may be used as an additive for animal feed.
The two types of processes have their respective advantages and disadvantages. In a wet milling process, the best advantage is that the separated components are of relatively high purity and the product streams can be diverted to several different commercial forms depending on market conditions. The biggest disadvantage is high equipment cost, which is only justifiable for large scale corn grinding operation. The second biggest disadvantage in wet milling is that it consumes large amounts of water. A wet milling operation in total uses upwards of 20 gallons of water per bushel. This increases operating costs because the water must be paid for, reclaimed and recycled.
Conversely for a dry milling process, the best advantage is relatively low cost for equipment and water usage. A dry milling operation uses approximately 0.1 gallons of water per bushel for the tempering process. Of course plant construction costs are increased with modified dry grind operations the more such modifications approach wet mill operations in terms of grain component separation. The biggest disadvantage in dry grind is limited product diversity due to lower purity in separations. To be cost effective, the majority of the revenue from a dry grind plant must be obtained from ethanol or other fermentation product produced, because the only product other than the one made by fermentation is the resulting DDG byproduct, which has relatively low value.
While both wet mill and dry mill operations have their respective advantages, they share a common disadvantage in that they incur a substantial cost in the consumption and handling of water. The steep water from a wet grind operation (corn steep liquor) contains organic acids, some volatile hydrocarbons, sugars, and minerals that cannot be discharged into the environment under most water regulation schemes. Those components must be removed by reclamation of the steep water, which may include use of a portion thereof as an additive in a fermentation process that will consume some of the materials in the corn steep liquor as a nutrient source. The tempering water used in a dry grind operation is typically carried with the corn grain through the dry milling and fermentation processes and not recovered until after distillation to make ethanol, where it is recovered as a the fraction of water remaining after distillation, and evaporation of the fermentation media, known as “backset” Backset is also obtained in wet dry operations where ethanol is made by fermentation of the starch. A portion of the backset water may be recycled for use in another round of fermentation, however, backset contains organic acids that inhibit growth of yeasts, and so only limited amounts of backset may be recycled. The rest must be cleaned.
It would be beneficial to develop a cost effective corn processing operation that did not rely on use of water, but that would also form products suitable for use as food, feed, fermentation and other industrial products at yields that would be economical in comparison to conventional dry grind and wet grind corn processing operations.